What is Soil-Cement? Advantages and Disadvantages

What is Soil-Cement? Advantages and Disadvantages

 

Soil-cement is a mixture of soil and measured amount of cement, and water compacted at high density. They all together form a hardened mixture like concrete, under the hydration action of cement.

Soil cement is used to strengthen underlying soil conditions to support traffic loading. Cement stabilized base is also a common application to strengthen the base section directly underneath rigid or flexible pavements. Soil-cement can be used when paving roads, parking lots, airports, residential streets, and more. It’s a cost-effective pavement base known for its strength and durability.

Soil-cement is also called the cement-stabilized base, or cement-treated aggregate base.

Soil-cement Applications

It is primarily used as a base course for-

  • Roads
  • Airports
  • Shoulders
  • Parking Areas

It is also used for:

  • Sub-base for rigid pavements
  • Slope protection for earth embankments and dams.
  • Foundation Stabilization
  •  Channel and Reservoir lining

How is soil-cement created?

First, rigorous laboratory tests are conducted to determine the cement and moisture content required to achieve the design compressive strength at a specific compactive effort. This analysis is then frequently referred to throughout the construction process to ensure that the soil-cement is of the highest quality.

Once the specific components of the mixture are decided, the material is mixed either in a central mixing plant or in-place. With central mixing plants, the soil-cement is first mixed and then brought to the job site. With in-place soil-cement construction, the mixing is done on-site. This involves first spreading the cement on the in-place soil. The cement, soil, and water are then mixed to a uniform consistency. During the final stages, the mixture goes through processes of compaction within a specified time limit and is cured. The curing process ensures that the soil-cement created is at its maximum strength.

Some soil-cement applications also require a process called “micro-cracking”. Micro-cracking of the soil-cement layer reduces the rigidity of the layer and the potential for cracking to reflect from the layer to the pavement.

Main processes involved in the construction of soil-cement are:

  • Mixing
  • Compacting
  • Curing
Mixing

At the central mixing plant, mixing of the soil-cement mixture is done. The final mixture is then moved to the job site and laid over the already prepared sub-grade level.

Initially, a proper quantity of cement is being spread over the soil and mixed homogeneously. Then a measured amount of water is added and mixed thoroughly. The mixing can be done by hands or with mixing equipment or machines.

Compaction

Now the compaction of the whole mixture is done by normally used compaction equipment. The compaction is done with high precision to achieve the maximum advantage of the cement used.

Once it is done, the whole mixture layer is cemented permanently at a very high density. After compaction, it won’t let the soil to undergo further consolidation or settlement under huge traffic.

Curing

The last step is curing. It is performed to prevent evaporation of water to the atmosphere. Proper cement hydration will be enabled only with an adequate amount of water. For this, a bituminous coating is laid over the layer and would act as a bituminous surface. The thickness of the layer can be increased if the pavement is constructed in an area with huge traffic.

 

Performance of Soil-cement

  • Soil-cement thicknesses are less in comparison of that required for granular bases carrying the same traffic over the same subgrade. It is because the soil-cement is a cemented, rigid material that distributes loads over broad areas. The slab-like characteristics and strength like beam are unmatched by granular bases. Hard and rigid soil-cement resists cyclic cold, rain, and spring-thaw damage.
  • The maintenance reports of already constructed soil-cement pavement show that they are having good service at low maintenance costs.
  • Samples must be taken at the installation and after a certain period. The analysis showed that the strength and other performance factors increased with age. It was found to be four times greater than the initial values. The analysis shows that they possess a high reserve strength capacity.

Advantage of Soil-cement

  • It requires less maintenance cost
  • It is Cheap and highly economical
  • Soil swelling can be reduced
  • It is Widely available resources.
  • Better weather-resistance and strength
  • It can be employed for small works

Disadvantages of Soil-cement

  • Formation of cracks
  • An always requirement to check the quantity of water
  • It is not suitable for some soils
  • Requirement of proper supervision.

 

What is a Buttress Dam ? Types, advantages and disadvantages

What is a Buttress Dam ? Types, advantages and disadvantages

 

A buttress dam is also called as a hollow dam. It is a dam with a solid, water-tight upstream side that is supported at intervals on the downstream side by a series of supports. A reinforced concrete slab or a series of arches or thickened buttress heads may form the sloping membrane for retaining the upstream water.  At the upstream end, a cut-off is provided to prevent or reduce the seepage of water.

 

What is Buttress?

 

A buttress is a thin wall of triangular profile shape. It has a characteristic of sloping upstream face. It is usually spaced at equal interval along the length of the dam and is supported either on a continuous mat foundation or on a separate spread footing. Each arch of the dam is supported by the buttresses. Multiple arch buttress dams are more durable and flexible than other buttress dams, such as a deck slab buttress dam.

Types of Buttress Dams

Deck slab Buttress Dam

The deck slab buttress dams are also known as Ambursen type buttress dams in honour of Nils F Ambursen who in 1903 built the first flat slab type of buttress dam. In this type of buttress dam, the sloping membrane or deck consists of a reinforced concrete slab supported by a series of buttresses. The inclination of the deck slab is kept between 40 to 55 degrees with the horizontal. In order to provide a wide base for the slab supported by the buttress the upstream end of the buttress where it joins the slab is usually made wide by providing haunch or corbel.

The deck slab buttress dam can be categorized into three types:

  • Fixed deck slab
  • Free deck slab
  • Cantilever slab

Each is having its own advantages and drawbacks. Advantage of the deck slab dam is that the deck slabs work together as a multiple arch dam for proper support. However, it is not like multiple arch dams, if one of the slabs gets damaged the damage will not affect the other slabs.

Buttress dams can be referred to as a gravity dam. The dam in which the concrete and material of the dam are designed to hold all the horizontal force and pressure of the water. Water pressure is distributed to the slabs of the Buttresses. Distributing the forces will reduce the load on the wall, allowing the dam to last longer.

 

Multiple arch Buttress dam

In this type of buttress dam, the sloping membrane or deck consists of a series of reinforced-concrete arches supported by a number of buttresses. The upstream face of the dam is usually inclined at 45. The arches are cast monolithically with the buttresses. Multiple arch buttress dams are more durable and flexible than other buttress dams, such as a deck slab buttress dam. The dam can be constructed as a singular stiffened wall or a double hollow one. The biggest disadvantage of the dam is that buttresses depend on each other. It means that if one buttress develops problems the whole dam will lose its efficiency. This dam is suitable for larger heights preferably above 50 meters.

 

Multiple Dome Buttress Dam

In this type of Buttress dam, the sloping membrane or deck consists of a series of reinforced-concrete domes supported by a number of buttresses. Most of the characteristics are the same as a multiple arch dam, however, instead of having arches it has domes.  Using multiple domes help to reduce the number of buttresses required to make the dam stable.  The main benefit is that the domes can be spaced farther apart than arches can be placed. It helps in reducing cost and saving material during designing of the dam.

 

Massive head Buttress dam

The main feature of this type of dam is, there is no separate water-retaining member is provided and the water-retaining member is formed only by enlarging the upstream end side of the buttress. Thus the dam is made with a series of buttresses and massive heads placed side by side. They are constructed with concrete mass and little reinforcement. This makes its construction relatively easy compared to other types of buttress dams. The weight of the concrete makes massive head buttress dams very heavy, and very resistant to sliding.

This type of buttress dam does not have a slab or arch at the upstream face like other buttress dams. Instead of having slab at the upstream face, the massive head buttress dams have buttress heads that are extended and connected with other buttress heads. The larger buttress heads can be made in different shapes i.e. round and diamond. These extensions of the buttresses are made stronger by using copper strips in the construction. The massive head buttress dam is resistant to sliding because of its weight.

 

Columnar Buttress dam

In this type of columnar buttress dam, the columns support the deck slab of the dam. The columns are inclined to the better support of the flat deck of the dam. The flat deck slabs are used to replace the buttresses. It is an altered deck slab buttress dam. It needs a very durable base. It requires skilled personnel to create buttresses. This is why it is not popular as much at the other types of buttress dams.

Advantages of Buttress dams

  1. The Buttress dam can be constructed on a relatively weak foundation.
  2. Buttress dams can be designed to fit in moderate amounts of foundation movement without serious damages, thus it can be built in the soil with differential settlements.
  3. The amount of concrete required for a buttress dam is about 1/3 to ½ of the concrete required for a gravity dam of the same height
  4. There is no problem with uplift or foundation drainage.
  5. The uplift pressure acting on a buttress dam is considerably less which leads to economy in the concrete and overall stability of the dam.
  6. The powerhouse, switchyard, etc., can be located between the buttresses thus saving some cost of construction.

Disadvantages of a Buttress dam

  1. Buttress dam requires more for work than solid concrete dams.
  2. As the thickness of the upstream concrete surface is less, it is more liable to get deteriorated.
  3. It requires constant maintenance and supervision.
  4. Life of the dam is less as compared to other dams.
  5. Skilled labour requirements and the shuttering to concrete ratio are greater for buttress dam than for a gravity dam. This may lead to a higher unit rate of concrete.
  6. The number of water seals to be provided and maintained for a buttress dam is usually more than for other dams.

 

 

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